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With the success of the World Wide Web and the Internet, data communication has become a part of daily life for many people. At the same time, voice communication and telephones still remain one of the most common techniques of communication. Understandably, sharing data and voice communication through the same media (e.g., telephone lines) has become an essential issue of Internet accessing techniques.
Different devices may share a copper twisted pair telephone line for transporting data and voice. For example, when a modem and a telephone share a telephone line, the modem transmits and receives computer data, while the telephone transmits and receives voice data.
There are two general approaches to address the sharing of data and voice communication through the same media. The first approach digitizes voice signals and treats voice signals as data (i.e., voice over Internet Protocol (IP)). The voice over IP approach transmits voice signals as digital packets using the Internet Protocol, rather than using protocols of the Public Switched Telephone Network (PSTN). This approach requires revolutionary change in the telephone infrastructure. More information about Voice over IP may be found, for example, in xe2x80x9cVoice over IP: Protocols and Standardsxe2x80x9d by Rakesh Arora, available on the World Wide Web at cis.ohio-state.edu/xcx9cjain/cis788-, which document is entirely incorporated herein by reference.
The second approach is to design modems (i.e., voice band modems) that can coexist with telephones while sharing a telephone line to transmit computer data and voice data. Voice band modems follow, for example, V32, V34, V90, V92, G.lite, or G.dmt protocol standards, which are available for purchase from the International Telecommunication Union and each of which is entirely incorporated herein by reference.
One of the critical problems in this second approach is reliable detection of the on-hook/off-hook status of a telephone handset connected in parallel with a modem on the same telephone line. At any given time, either the modem or the telephone exclusively transmits or receives data. For example, when a user picks up a telephone handset to make or answer a telephone call, the telephone is said to be off-hook. When the telephone handset is off-hook, voice data is being transmitted or received over the telephone line, and the modem cannot transmit or receive data. When the user places the handset back on the telephone, the telephone is said to be on-hook. At this time, the modem can transmit or receive data. Similarly, if a modem is transmitting or receiving data, the modem is said to be off-hook, and when the modem is not transmitting or receiving data, the modem is said to be on-hook. While the modem is off-hook, a user may not be able to make or answer a telephone call.
One technique for detecting the status of a parallel-connected telephone handset measures the change in the direct current (DC) line current. For more information, see U.S. Pat. No. 6,240,177 B1 (the ""177 patent), entitled xe2x80x9cCommunication Device Comprising an Off-Hook Detection Circuit,xe2x80x9d issued on May 29, 2001 to Guntzburger et al. The ""177 patent describes a communications device (e.g., a modem) that is connected in parallel with at least one item of communications apparatus (e.g., a telephone handset) on a transmission line. (Abstract; Col. 3, lines 25-30) The strength of the DC component of the current flowing in a modem is monitored. (Col. 4, lines 20-22) A variation of the current strength is detected when the item of apparatus connected to the transmission line is unhooked. (Col. 4, lines 22-25)
The change in DC current technique is a hardware-based solution. A disadvantage of this change in DC current technique is that the modem requires detection hardware or circuits, which limits the precision and flexibility of detection. For example, if a modem has a detector capable of detecting a 3 mA change in DC line current, the modem will fail to identify the on-hook or off-hook status of a handset if the handset changes the line current by less than 3 mA when going on-hook or off-hook. The precision of the line current detection can be enhanced to meet current sensing requirements when line loads change, but that could cause one hardware revision after another, which is expensive for the modem and creates upgrade or compatibility problems for modem owners.
Another technique for determining on-hook/off-hook status of parallel devices measures the changes in channel response. This is further described in xe2x80x9cG.Lite: Effect of Telephony Equipment on Line Insertion Lossxe2x80x9d by Ashley Pickering, presented to the ITU-Telecommunication Standardization Sector in Waikiki, Hi., Jun. 29-Jul. 3, 1998, which is entirely incorporated herein by reference. Disadvantages of this technique are the higher computational power required for periodically checking channel response and the complexity of interpreting the determined response to identify whether a parallel device is on-hook or off-hook.
In accordance with an embodiment of the invention, a computer having a modem is programmed with software (also called a xe2x80x9con-hook/off-hook detectorxe2x80x9d) that detects a change in status (e.g., on-hook or off-hook) of a device (e.g., a telephone having a handset) by measuring a change in the power of a signal flowing through a connection between the modem and the device. In one example, the computer programmed with the on-hook/off-hook detector detects a status of a telephone handset by obtaining a measure of a reference signal power and obtaining a measure of an instantaneous signal power on the connection and comparing these two measurements. The reference signal is a constant signal whose power does not change over time. Then, the computer programmed with the on-hook/off-hook detector is able to detect whether there has been a change in status of the telephone handset by comparing the detected (xe2x80x9cinstantaneousxe2x80x9d) status with a previous status of the telephone handset. The on-hook/off-hook detector may be executed in a central processing unit (CPU) of a computer or in a digital signal processor (DSP) of a modem.